Mechanisms behind active trends in body size evolution of the Canidae (Carnivora: Mammalia).

Clade-level interactions can impart trends on observed characters. In this study, origination and extinction events were compiled for the three subfamilies of the Canidae (dogs): Hesperocyoninae, Borophaginae, and Caninae. These events were binned into 2-million-year time slices, and median body masses were calculated for each time slice. Originations and extinctions were classified as "large" or "small" relative to these median values. Likelihood ratios demonstrate that originations were significantly biased from random in four time slices, while extinctions were never significantly different from random. Two models were chosen by the Akaike Information Criterion to describe the evolutionary dynamics of canid body size. Both proposed a consistent, significant bias toward larger originations, punctuated by episodic "pulses" of increased bias toward larger originations and a Quaternary "pulse" favoring smaller originations, tracking the Quaternary diversification of foxlike canids. These pulses correspond in each subfamily to periods of rapid taxonomic diversification and the appearances of morphologies associated with hypercarnivorous diets. Together, the diversity data, the appearance of hypercarnivory, and the biases in originations document competitive interactions between Hesperocyoninae and Borophaginae and suggest that body size trends were driven, in part, by clade-level dynamics in the Canidae.